The alkylisocyanates liberated from BCNU and NCNU and active-site directed inhibitors of the enzyme transglutaminase (Factor XIII in plasma). We have previously reported that these drugs keep the enzyme in the organs of the mice inactivated up to 16 hours after administration. Our explanation for the cancerostatic action of these nitrosoureas is that these compounds inactivate transglutaminase which carries out the cover-up process for the proliferating cells. It can be shown that in the cytotoxic test, release of 51Cr from the targets is inhibited if fibrinogen molecules are attached to the cell surface with the help of transglutaminase. Such masked cells are even more vicious when implanted into mice and are poor targets for killer lymphocytes.